This section provides background information related to the present disclosure which is not necessarily prior art.
Reverse osmosis (RO) systems use special membranes to separate a feed stream containing dissolved materials such as salt into two streams; one stream containing purified water called permeate and the other stream containing a concentrated solution of dissolved materials called concentrate or brine.
The amount of pressure to drive the separation process is strongly dependent on the concentration of dissolved solids in the feed stream. For fluids such as seawater, the minimum pressure required to drive the separation process can be on the order of 500 to 600 pound-force per square inch gauge (psig), but would typically be as high as 900 psig to achieve reasonable rates of permeate production. Such high pressure requires a great deal of energy to be expended by a high pressure pump (HPP).
In addition to the high pressure requirement, the RO process can only extract a limited amount of permeate from the feed stream. In the case of seawater RO process, typically 40 to 45% of the feed stream can be extracted as permeate with the balance discharged as concentrate waste.
The pressure of the permeate stream is low as its passage through the membrane absorbs the available pressure. The pressure of the concentrate stream, which did not pass through the membrane surface, remains very close to the feed pressure. Therefore, in the case of seawater, the concentrate pressure is very high.
Referring now to FIG. 1, a reverse osmosis system 10 according to the prior art includes a membrane array 12 that generates a permeate stream 14 and a brine stream 16 from a feed stream 18. The feed stream 18 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream 18 to a required pressure, and the feed stream 18 enters the membrane array 12 at the required pressure.
The membrane array 12 includes a membrane housing 24 and a membrane 26. The portion of the feed stream 18 that flows through the membrane 26 before exiting the membrane array 12 forms the permeate stream 14. The portion of the feed stream 18 that does not flow through the membrane 26 before exiting the membrane array 12 forms the brine stream 16.
The permeate stream 14 is purified fluid flow at a low pressure. The brine stream 16 is a higher pressure stream that contains dissolved materials blocked by the membrane 26. The pressure of the brine stream 16 is only slightly lower than the feed stream 18. A control valve 28 may be used to regulate the flow through and pressure in the membrane array 12. The brine stream 16 may flow through the control valve 28 and to a drain 30.
Referring now to FIG. 2, a reverse osmosis system 50 according to the prior art is similar to the reverse osmosis system 10 of FIG. 1 except that the reverse osmosis system 50 includes an additional membrane array 52. The membrane array 52 generates a permeate stream 54 and a brine stream 56 from the brine stream 16 exiting the membrane array 12. The permeate stream 54 may be joined with the permeate stream 14 to form a single permeate stream 58.
The membrane array 52 includes a membrane housing 60 and a membrane 62. The portion of the brine stream 16 that flows through the membrane 62 before exiting the membrane array 52 forms the permeate stream 54. The portion of the brine stream 16 that does not flow through the membrane 62 before exiting the membrane array 52 forms the brine stream 56. The brine stream 56 flows through the control valve 28 and to the drain 30.
Referring now to FIG. 3, a reverse osmosis system 100 according to the prior art is similar to the reverse osmosis system 10 of FIG. 1 except that the reverse osmosis system 100 includes a hydraulic pressure booster 102. The booster 102 is used to recover hydraulic energy from the brine stream 16 and includes a pump portion 104 and a turbine portion 106. The pump portion 104 and the turbine portion 106 are coupled together with a common shaft 108. The brine stream 16, at a high pressure, passes through the turbine portion 106, which causes the shaft 108 to rotate and drive the pump portion 104. After passing through the turbine portion 106, the brine stream 16 is at a low pressure and flows to the drain 30.
The pump portion 104 increases the feed pressure in the feed stream 18. The booster 102 generates a portion of the feed pressure requirement for the membrane array 12 and, thus, the feed pump 20 and the motor 22 may be reduced in size since a reduced amount of pressure is required by them. In addition, the amount of energy consumed by the feed pump 20 and the motor 22 will be reduced.
Referring now to FIG. 4, a reverse osmosis system 150 according to the prior art is similar to the reverse osmosis system 50 of FIG. 2 except that the reverse osmosis system 150 includes a hydraulic pressure booster 152. The booster 152 is used to recover energy from the brine stream 56 and includes a pump portion 154 and a turbine portion 156. The pump portion 154 and the turbine portion 156 are coupled together with a common shaft 158. The brine stream 56, at a high pressure, passes through the turbine portion 156, which causes the shaft 158 to rotate and drive the pump portion 154. After passing through the turbine portion 156, the brine stream 56 is at a low pressure and flows to the drain 30.
The pump portion 154 increases the pressure of the brine stream 16 before the brine stream 16 enters the membrane array 52. The amount of solids dissolved in the brine stream 16 may be greater than the amount of solids dissolved in the feed stream 18. Thus, it may be necessary to increase the pressure of the brine stream 16 to a higher pressure relative to the pressure of the feed stream 18. A motor-driven, interstage pump (not shown) may be included between the membrane arrays 12, 52 to increase the pressure of the brine stream 16 to this higher pressure. However, since the booster 152 is driven by the hydraulic energy in the brine stream 16, the booster 152 may consume less energy relative to the interstage pump. In addition, using the booster 152 may allow the pressure of the feed stream 18 to be reduced. Thus, the feed pump 20 and the motor 22 may be reduced in size since a reduced amount of pressure is required by them. In addition, the amount of energy consumed by the feed pump 20 and the motor 22 may be reduced.